The CDMA (Code Division Multiple Access) system has the potential of increasing the subscriber capacity with respect to the radio frequency and attracts attention as a radio access system for a mobile communication cellular system.
In the CDMA system, however, has a problem in that, when a base station receives a desired signal from a particular user, interference occurs with other signals from users who access the same base station simultaneously with the particular user. To eliminate such interference signals to properly receive a desired signal, an array antenna is usable.
An array antenna comprises a plurality of antenna elements. The array antenna weights each antenna element by a complex number to control the amplitude and phase of a signal received by each antenna element, thereby forming a directional beam. With the directional beam formed to be suitable for receiving a desired signal, the array antenna can properly receive the desired signal and also suppress interference signals from other users. An adaptive array antenna is the one that adaptively controls such a directional beam.
In a base station, desired signals are received through a multipath channel. The adaptive array antenna directs high beam gains to the respective path directions of desired signals as well as directing a point (null) where the gain is extremely low to the direction of an interference signal to control weighting operation so that SINR (Signal to Interference and Noise Ratio) is maximized.
There is described a conventional adaptive antenna receiver in Non Patent Document 1.
FIG. 1 is a block diagram showing the construction of a conventional adaptive antenna receiver. Referring to FIG. 1, the conventional adaptive antenna receiver comprises path receivers 101-1 to 101-L (L: a positive integer), a combiner 106, a determination unit 107, a switch 108, and a subtractor 109.
There are provided L pieces of the path receivers 101-1 to 101-L to perform multipath combining correspondingly to a plurality of multipath transmission channels in a mobile communication environment.
All of the path receivers 101-1 to 101-L have the same construction.
The path receivers 101-1 includes a beamformer 102-1, a transmission channel estimation section 103-1, a complex conjugate operation section 104-1, multipliers 105-1 and 110-1, and an antenna weight adaptive update section 111-1.
The beamformer 102-1 receives as input N despread signals obtained by despreading signals received by respective antenna elements #1 to #N (N: the number of antenna elements included in an array antenna) in path #1 timing with the spreading code of a desired user. The beamformer 102-1 performs the weighting and combining of the N despread signals to form a directional beam for the path #1.
FIG. 2 is a block diagram showing the construction of the conventional beamformer. Referring to FIG. 2, the beamformer 102-1 includes complex conjugate operation sections 112-1-1 to 112-1-N, multipliers 113-1-1 to 113-1-N, and a combiner 114-1. The other beamformers 102-2 to 102-L each have the same construction as described above.
The complex conjugate operation sections 112-1-1 to 112-1-N calculate the complex conjugates of N antenna weights (W) obtained from the antenna weight adaptive update section 111-1, respectively, and feed them to the multipliers 113-1-1 to 113-1-N.
Each of the multipliers 113-1-1 to 113-1-N multiplies each despread signal of the path #1 by the complex conjugate of the antenna weight fed from corresponding one of the complex conjugate operation sections 112-1-1 to 112-1-N, and feeds the product to the combiner 114-1.
The combiner 114-1 adds up all the output of the multipliers 113-1-1 to 113-1-N.
Besides, the transmission channel estimation section 103-1 shown in FIG. 1 performs transmission channel estimation based on the output of the beamformer 102-1 to feed a transmission channel estimation value to the complex conjugate operation section 104-1 and the multiplier 110-1. Incidentally, the transmission channel estimation indicates the estimation of changes in radiowave propagation condition based on the path reception state. The transmission channel estimation value thereby obtained is used to compensate the changes in radiowave propagation condition (transmission channel correction).
The complex conjugate operation section 104-1 calculates the complex conjugate of the transmission channel estimation value received from the transmission channel estimation section 103-1 to feed it to the multiplier 105-1.
The multiplier 105-1 multiplies the output of the beamformer 102-1 by the complex conjugate of the transmission channel estimation value to correct phase variation (transmission channel correction) as well as to perform weighting operation for maximum ratio combining. The maximum ratio combining is such a weighted combining method to maximize The SINR of a signal after multipath combining.
In the conventional path receivers 101-1 shown in FIG. 1, the function for phase correction based on the transmission channel estimation is separated from the antenna weight control. Therefore, in the antenna weight control, there is no need to correct phase variation caused by the phasing of a desired signal, and it is just required to correct phase variation depending only on the arrival direction of the signal. Thus, stable beamforming can be performed.
The combiner 106 adds up all the output of the multipliers 105-1 to 105-L of the respective path receivers 101-1 to 101-L to perform path combining, thereby-generating a demodulation signal.
The determination unit 107 determines a transmission symbol with the highest possibility based on the demodulation signal obtained by the combiner 106.
The switch 108 selects either a known reference signal or the transmission symbol from the determination unit 107 to feed the selected one as a reference signal to the subtractor 109. When having been provided with a known reference signal, the switch 108 selects the signal. On the other hand, when having been provided with no known reference signal, the switch 108 selects the transmission symbol from the determination unit 107.
The subtractor 109 subtracts the demodulation signal generated by the combiner 106 from a reference signal, and feeds the difference as an error signal to the multipliers 110-1 to 110-L of all the path receivers 101-1 to 101-L. On this occasion, the subtractor 109 uses as the reference signal a value obtained by multiplying the reference signal from the switch 108 by a reference signal level, which will be described later.
The multiplier 110-1 of the path receiver 101-1 multiplies the error signal from the subtractor 109 by the transmission channel estimation value from the transmission channel estimation section 103-1 to feed the product to the antenna weight adaptive update section 111-1.
The antenna weight adaptive update section 111-1 adaptively calculates the antenna weight based on the error signal multiplied by the transmission channel estimation value and the N despread signals of the path #1, and feeds the calculation result to the beamformer 102-1 to perform the adaptive control. In general, minimum mean square error (MMSE) control is used for the adaptive control. As adaptive update algorithms for antenna weighting factors using error signals, LMS (Least Mean Square), NLMS (Normalized LMS), and RLS (Recursive Least Square) algorithms are known.
For example, in Non Patent Document 1, there is described a technique in which the antenna weight is updated with the NLMS algorithm. The antenna weight w(i, m) (where i is the path number, and m is the symbol number) can be calculated by the following update equation (Equation 1):
                    (                  Equation          ⁢                                                            ⁢                                                          ⁢          1                )                                                                      w          ⁡                      (                          i              ,                              m                +                1                                      )                          =                              w            ⁡                          (                              i                ,                m                            )                                +                                    λ                              p                ⁡                                  (                                      i                    ,                    m                                    )                                                      ⁢                          x              ⁡                              (                                  i                  ,                  m                                )                                      ⁢                          h              ⁡                              (                                  i                  ,                  m                                )                                      ⁢            e            *                          (              m              )                                                          (        1        )            
where x(i, m) is the despread signal of a signal received by each antenna, p(i, m) is the total power of the despread signals of the respective antennas, h(i, m) is the transmission channel estimation value, and λ is the step size. Incidentally, * is the conjugate complex number.
If an error signal is denoted by e(m) and a received signal is denoted by z(m), then, the error signal e(m) can be expressed by the following Equation (2):e(m)=A(m){circumflex over (z)}(m)−z(m)  (Equation 2)where {circumflex over (z)}(m) is the reference signal (a known reference signal or determination signal), and A(m) is the reference signal level.
The reference signal level A(m) is calculated based on the despread signal of each antenna element input to the beamformer 102-1 (e.g., see Patent Document 1). The reception level of the despread signal input to the beamformer 102-1 is not affected by the beam gain, and desirable as a reference to calculate the reference signal level A(m).
In an application of the adaptive antenna receiver, a CDMA (Code Division Multiple Access) signal is received by a plurality of array antennas (sub-arrays) whose phasings are independent of one another to achieve the directivity control effect and the diversity effect.
There has been proposed a technique in which a common antenna weight is used for directivity forming in the respective sub-arrays and adaptive control characteristics are improved by applying the determination error signals of all the sub-arrays to the antenna weight control (see Patent Document 2).
In another application of the adaptive antenna receiver, a signal of a beam formed by a multi-beamformer is used for path detection. Thereby, path detection characteristics are not deteriorated even if there are a large number of antennas.
There has been proposed a technique in which the despread output of a beam formed by a multi-beamformer is weighted and combined for correcting phase variation to thereby detect each path. The paths are combined to obtain a demodulation signal. The weight used for weighted combining is adaptively updated based on the determination error signal obtained by the inverse correction of the phase variation and the beam despread output (see Patent Document 3). With this technique, the excellent path detection characteristics and reception demodulation characteristics can be realized.
Patent Document 1: Japanese Patent Laid-Open No. 2002-77008
Patent Document 2: Japanese Patent Laid-Open No. 2002-368520
Patent Document 3: Japanese Patent Laid-Open No. 2002-368652
Non Patent Document 1: Tanaka, Sawahashi, Adachi, et al., “Pilot Symbol-Assisted Decision-Directed Coherent Adaptive Array Diversity for DS-CDMA Mobile Radio Reverse Link”, IEICE Trans., vol. E80-A, pp. 2445-2454, December 1997